It is recognized in the prior art that contact operating mechanisms must quickly respond to severe fault conditions by permitting the contact arms and associated contacts to blow open quickly in response to unacceptable fault current conditions. Various mechanical arrangements have been provided to improve blow-open response speeds for the current carrying contacts. For example, circuit breaker designs have produced circuit breaker configurations which provide current paths which utilize the high currents during fault conditions to increase electromagnetic blow-apart forces between contact arms and associated current carrying members in the circuit breaker. Unacceptable fault currents within properly configured current paths produce electrodynamic forces which drive contact arms and associated contacts open quickly. One problem encountered as a result of high blow-apart forces is the potential that these forces cause the contact arm to rebound from its associated stop and bounce back past the over-center position, thereby causing the arm to reengage the circuit breaker contacts.
Another type of circuit breaker design practice used to improve the separation speed and performance of the circuit breaker contacts during unacceptable fault current conditions is the use of cam arrangements with the contact arm. These arrangements produce high contact forces to produce adequate contact engagement pressures during normal operation, and produce reduced or eliminated contact force after the contact arm is moved a relatively small distance during blow-apart under fault current conditions. For example, U.S. Pat. No. 4,488,133, issued to McClellan et al. on Dec. 11, 1984, discloses a current-limiting circuit breaker including a contact pressure spring configured to hold the contact arms and associated contacts in their open position after the contacts are blown open. In particular, a multi-section cam transmits contact opening and closing forces produced by a spring powered, over-center, toggle-type operating mechanism to the associated pivoting contact arms. A follower on the contact arm is biased into engagement with the cam by the contact pressure spring. The cam is configured so that the moveable contact arm requires relatively little motion to move the knee of the cam surface into the open direction during blow-off under fault conditions. The cam is further configured to control the speed of the contact arm to reduce the potential for contact arm rebound.
While various designs have been provided to increase blow-off speeds and performance of circuit breaker contact arms during unacceptable fault current conditions, the need to reduce circuit breaker size and cost for given voltage and current carrying ratings requires that blow-off speeds and performance increase even though there is substantial market pressure to maintain present circuit breaker selling prices. Accordingly, it would be desirable to provide a design which is an improvement over the above-described designs and lower in cost.